1. Field of Invention
This invention relates to antennas. Specifically, the present invention relates to helical antennas.
2. Description of the Related Art
Helical antennas are well known in the art. See for example U.S. Pat. No. 5,541,617 issued Jul. 30, 1996, to Connolly et al.; U.S. Pat. No. 5,349,365 issued Sep. 20, 1994 to Ow et al.; U.S. Pat. No. 5,134,422 issued Jul. 28, 1992 to Auriol; U.S. Pat. No. 4,349,824 issued Sep. 14, 1982 to Harris; U.S. Pat. No. 5,255,005 issued Oct. 19, 1993 to Terret et al.; U.S. Pat. No. 5,170,176 issued Dec. 8, 1992 to Yasunaga et al.; and U.S. Pat. No. 5,198,831 issued Mar. 30, 1993 to Burrell et al., the teachings of which are hereby incorporated herein by reference. See also "A Shape-Beam Antenna For Satellite Data Communication" published Oct. 12, 1976, by Randolph W. Bricker, Jr. AP-S Session 4, 1630, at the AP-S. International Symposium held in 1976 in Amherst, Mass., U.S.A., pp. 121-126.
As noted by Auriol, helical antennas offer the advantage of radiating an electromagnetic wave in a high-quality circular polarization state over a wide coverage area with a transmission lobe which may be shaped as needed for a given application. These characteristics make helical antennas valuable in various fields of use, such as ground links with orbiting satellites or mobile/relay ground links with geosynchronous satellites.
Popular receiving helical antennas are typically either bifilar with two helices spaced equally and circumferentially on a cylinder or quadrifilar with four helices arranged the same way. Because of the radiation or coverage pattern thereof, quadrifilar helix antennas are typically well suited for mobile-to-satellite communication applications. As discussed in Antenna Engineering Handbook by Richard C. Johnson and Henry Jasik, pp. 13-19 through 13-21 (1984), a quadrifilar helix (or volute) antenna is a circularly polarized antenna having four orthogonal fractional-turn (one fourth to one turn) helixes excited in phase quadrature. Each helix is balun-fed at the top with four helical arms of wires or metallic strips of resonant lengths (I=.lambda./4, m=1, 2, 3, . . . ) wound on a small diameter with a large pitch angle. This antenna is a fairly well suited for various applications requiring a wide hemi-spherical or cardioid shaped radiation pattern. In addition, quadrifilar helix antennas generally offer a high bandwidth as compared to patch antennas over the high frequency ranges required for satellite communication (e.g., GPS) applications.
Recently, a need has been recognized for an antenna suitable for use in mobile satellite radio applications. For the reasons set forth above, the quadrifilar helix antenna is a prime candidate. One of the advantages of the quadrifilar antenna is its compact size and relatively small diameter. For the satellite radio application, the height of the antenna must conform to size and space constraints for a target environment (e.g. automobile installation). Unfortunately, as is well known in the art, the height of a quadrifilar helix antenna is directly related to its impedance. Consequently, any change in the height of the antenna will affect its impedance and its performance. Hence, changes in height of conventional quadrifilar helix antennas typically require a redesign of the impedance matching circuit associated therewith and may affect other components of the system in which it is installed as well.
In addition, changes in the height of conventional quadrifilar helix antennas are limited in that the height of the antenna, that is, the length of the radiating elements, must be a discrete integer multiple of one quarter-wavelength (.lambda./4) of the operating frequency of antenna. Further such reductions in the height of conventional quadrifilar helix antennas are achieved, generally, at the cost of reduced gain.
Therefore, for certain applications, a need exists in the art for a system or method for variably adjusting the height of helical antennas, particularly quadrifilar helix antennas, without affecting the performance of same.